1. Field of the Invention
The present invention relates to a white organic light emitting device. More specifically, the present invention relates to a tandem white organic light emitting device, efficiency, voltage and lifetime of which are improved by changing a configuration of a charge generation layer.
2. Discussion of the Related Art
The recent trend toward information-dependent age has brought about rapid development in display fields that visually display electrical information signals. In this regard, a variety of flat display devices having superior properties such as slimness, light weight and low power consumption have been developed and are actively used as substitutes for conventional cathode ray tubes (CRTs).
Specific examples of flat display devices include liquid crystal display devices (LCDs), plasma display panel devices (PDPs), field emission display devices (FEDs), organic light emitting devices (OLEDs) and the like.
Among these, organic light emitting devices that do not require an additional light source, realize a compact design and render clear color are considered to be competitive applications.
Organic light emitting display devices require formation of organic light emitting layers. The formation of organic light emitting layers is generally carried out by a deposition method using a shadow mask.
However, a large-area shadow mask may be bent due to load. For this reason, it is impossible to use the large-area shadow mask several times, and defects associated with pattern formation of organic light emitting layers occur. Accordingly, there is a demand for an alternative method.
A tandem organic light emitting device (hereinafter, referred to as “tandem organic light emitting device”) suggested as an alternative to the shadow mask will be described below.
The tandem organic light emitting device is characterized in that respective light emitting diode layers interposed between an anode and a cathode are deposited without using a mask, and organic films including the organic light emitting layers are sequentially deposited under vacuum using different materials.
Meanwhile, realization of white light using a tandem organic light emitting device may be performed by mixing light emitted from two or more light emitting layers. In this case, the tandem organic light emitting device includes a plurality of light emitting layers that emit light with different colors that are interposed between an anode and a cathode, and a charge generation layer (CGL) provided between respective light emitting layers. Stacks are separated, based on each light emitting layer.
In such a tandem organic light emitting device, one material does not emit light and a plurality of light emitting layers containing light emitting materials having different photoluminescence peaks (PL peaks) at respective wavelengths emit light at different positions of the device and the light is combined to realize light emission.
However, the conventional tandem organic light emitting device has the following problems.
The tandem device includes a charge generation layer between different stacks. Also, the charge generation layer transports electrons or holes to an adjacent stack. In this regard, when a charge generation layer is formed using the material commonly used for the tandem device, transport of electrons or holes between the adjacent stack and the charge generation layer may be impossible due to poor interface properties of the stacks adjacent to the charge generation layer. In this case, emission of light may normally occur in the adjacent phosphorescent or fluorescent stack.
Accordingly, a great deal of research to find factors that block injection of holes or electrons from the charge generation layer to adjacent stacks is underway.